Most trials have not been exclusionary enough; this has created difficulty in finding a drug that actually works. Tinnitus is a symptom; not a disorder. It has a cause. Noise induced tinnitus, drug induced tinnitus, TMJ induced tinnitus, Eustachian Tube Dysfunction induced tinnitus, stress induced tinnitus, bilateral tinnitus, unilateral tinnitus, and so fourth.
If they are going to find a drug that helps at least one of those previously described groups they have to narrow down the inclusion criteria. Drug XX might only help those with noise induced tinnitus. etc.
Remember, a big part of the problem of tinnitus is that it is so broad and can arise from so many pathologies. Many previous studies have included tinnitus sufferers from many groups and as such the drugs effectiveness may have been misrepresented.